Damage and theft are common problems with boat propellers. Damage may occur during boat transport, boat storage or impact with submerged objects, lake or river bottoms. Impacts result in damaged propeller blades and out-of-balance conditions. Corrosion occurs during outdoor storage as well as submersion in water.
Replacement propellers are high in cost because of their complex shapes, precision balance and corrosion resistance requirements. Precision balance requires expensive machining. Corrosion resistance requires expensive materials and coatings.
Theft is a major problem because of the high costs of replacement propellers and the ease of propeller removal. It is not unusual for a replacement propeller to cost several hundred dollars. Propellers are attached with standard nuts making them easily removable with common tools. The high costs and ease of removal make them attractive to thieves.
Anti-theft devices for boat propellers are known in the art. One disadvantage of known devices is that they are not applicable to existing engines. Macchi U.S. Pat. No. 3,732,033; Reese U.S. Pat. No. 3,759,076; and Sims U.S. Pat. No. 4,257,247 are exemplary of the prior art. Macchi discloses a lockable cover for enclosing a nut which retains a propeller. Reese discloses a locking bar for locking a propeller to a hub. Sims discloses a cage-like structure, chain and pad lock for preventing removal of a castellated nut which retains a boat propeller. All of the above require engine modifications.